In the days of sail, naval vessels mounted cannon carefully and purposefully rigged into positions which optimised the effect of their fire. Occasionally, the rigging of these very heavy guns might come loose, and with the vessel pitching about on high seas or during manuever in battle, the results for the ship and crew could prove catastrophic. The term has come to describe a person lacking prudence or insight, whose actions and/or speech jeopardises the safety of people in their proximity or under their authority.

Like a drunken bully, the politician shrugged off his advisors and taunted the insurgent forces operating in the country his armies were attempting to pacify. He had become a LOOSE CANNON.

a) (offensive) an irrational/non pragmatic/behaviorally unpredictable person who is neither under the influence of any drugs/alcohol nor has ADD/ADHD
b ) (sarcastic) a person who does rational and somewhat pragmatic things first without, however, having thought them through properly and then asks intelligent questions regarding his previous pragmatic actions
c) an offensive term for children with, or perceived to have, ADD/ADHD, regardless of whether or not this is truly the case

boy a) my girlfriend is such a loose cannon, and makes the most irrational, non-pragmatic decisions, is there anything I can do?

boy b) are you lol, your girlfriend's social worker or psychiatrist? It may be that she is just a loose cannon according to you, IYHO only and normal to everyone else. and lol, is it possible that you are a loose cannon according to her?

boy a) Dunno and idc if i am a loose cannon according to her. IMHO she needs a few doses of methylphenidate